The present invention is directed to a method of treating a subterranean formation penetrated by a well bore utilizing liquid carbon dioxide admixed with a substantially anhydrous, viscosified fluid.
The treatment of subterranean formations penetrated by a well bore to stimulate the production of hydrocarbons or the ability of the formation to accept injected fluids has long been known in the art. One of the most common methods of increasing productivity of a hydrocarbon-bearing formation is to subject the formation to a fracturing treatment. This treatment is effected by injecting a liquid, gas or two-phase fluid which generally is referred to as a fracturing fluid down the well bore at sufficient pressure and flow rate to create a crack or fracture, in the subterranean formation. A proppant material such as sand, fine gravel, sintered bauxite, glass beads or the like can be introduced into the fractures to keep them open. The propped fracture provides larger flow channels through which an increased quantity of a hydrocarbon can flow, thereby increasing the productive capability of a well.
Treatment of subterranean formations using carbon dioxide based fluids is also known in the art. Fluids containing carbon dioxide are desirable for certain well conditions because the fluids are believed to be nondamaging. In prior patents, carbon dioxide based fluids have typically been formed in combination with aqueous liquids or substantially anhydrous liquids. Methods of stimulating wells using such carbon dioxide based fluids are described in U.S. Pat. Re. 32,302; 4,519,455; 4,627,495; and 4,554,082 which are herein incorporated by reference.
Aqueous fluids have been traditionally used in conjunction with carbon dioxide because liquid carbon dioxide alone does not have adequate viscosity to effectively treat a well. U.S. Pat. Re. 32,302 discloses methods and fluids for fracturing subterranean formations by utilizing liquid-liquid emulsions of liquified carbon dioxide and an aqueous fluid. The liquid-liquid emulsion contains a surfactant to stabilize the emulsion and the resulting foam when the emulsion is heated above the critical temperature of carbon dioxide. The fluid may also contain a gelling agent, such as a water-soluble synthetic or natural polymer, for additional stability. U.S. Pat. No. 4,627,495 also discloses methods of fracturing subterranean formations using liquified carbon dioxide and an aqueous liquid which contains a surfactant stabilizer.
Certain well conditions require that a substantially anhydrous fluid be used for treatment. U.S. Pat. Nos. 4,554,082 and 4,519,455 disclose methods and compositions for fracturing a subterranean formation using stabilized liquid-liquid emulsion of liquified carbon dioxide and substantially anhydrous liquid. Examples given of substantially anhydrous fluids are kerosene, diesel, light crude oils, and mixtures thereof with glycols.
In U.S. Pat. Nos. 3,765,488 and 3,954,626, methods and compositions of treating a subterranean formation using liquified carbon dioxide and viscosified alcohols are also disclosed. Hydroxyalkyl cellulose and hydroxyalkyl methyl cellulose are used to viscosify essentially anhydrous alcohols having 1 to 3 carbon atoms. The liquified carbon dioxide is admixed with the viscosified alcohol and injected down the well bore. The disclosed treating fluids contain liquified petroleum gas (LPG) and the importance of the mixture ratio of carbon dioxide to LPG to gelled alcohol on the critical properties of the fluid is also disclosed.
U.S. Pat. No. 4,609,043 discloses an enhanced oil recovery process in which liquified carbon dioxide is injected into the oil-bearing formation under supercritical conditions to act as a solvent for the oil. Mobility of the carbon dioxide is controlled by the use of a dissolved polymer whose solubility is enhanced by the use of an "entrainer". The dissolved polymers disclosed are synthetic polymers such as a tactic polybutene and polypropylene oxide, polyvinylethyl ether, poly n-decylacrylate and poly n-laurylmethacrylate. Slightly polar, low molecular weight organic compounds ("entrainer") are disclosed for increasing the solubility of polymers in the carbon dioxide. Preferred entrainers are disclosed, such as alcohols (methanol, ethanol, propanol and butanol), glycols and polyglycols.
In U.S. Pat. No. 4,567,947, fracturing fluid compositions are disclosed comprising at least one substantially anhydrous aliphatic alcohol, a non-ionic homopolymer of polyalkylene oxide and a gel activating agent. Mixing the composition with liquid carbon dioxide in a concentration of from 40% to 80% is also disclosed. The polyalkylene oxide homopolymer disclosed has a molecular weight from about 4 to about 5 million and the gel activating agent is an alkali metal halide or an alkaline earth metal halide.
In general, the fluids or treating fluid compositions discussed above are introduced into the subterranean formation through a well bore. The temperature and pressure of the fluid is controlled to maintain the carbon dioxide in the liquid phase during injection. Once in the well bore, the fluid is slowly heated to a temperature above the critical temperature of carbon dioxide, i.e., about 88.degree. F. As the liquid carbon dioxide undergoes conversion to a gas, a slight increase in the volume of the carbon dioxide occurs. The term "gas" as used herein means a fluid at a temperature equal to or above the critical temperature of the fluid while maintained at any given pressure. Upon conversion of the liquid carbon dioxide to a gas, a stabilized foam is formed spontaneously.
The methods and fluids of the present invention are an improvement over the methods discussed above in many ways. First significant viscosity enhancement of the carbon dioxide is achieved by the fluid of the present invention. Second, a single-phase fluid can be formed with liquid carbon dioxide. Finally, the surprising discovery has been made that high proportions of liquid carbon dioxide are miscible with polymers described herein.